Epistemic closure or deductive closure is the philosophical concept that we know all of the consequences of those things that we know. That is, that our knowledge is "closed" with respect to "consequent".

e.g. If epistemic closure were true in mathematics then, if we knew a set of axioms, we would know all of the theorems which follow from the axioms. In propositional logic, this is true for the set of all propositions.

It is fairly obvious that knowledge is not closed under consequence as described above. There are, however, weaker versions of the closure principle which are more plausible. For instance, knowledge could be closed under known consequence; that is, if an agent A knows that p and knows that p implies q, then A either knows q or (weaker still) is in a position to know q.

Extension to politics

Julian Sanchez of the Cato Institute adapted the term "epistemic closure" to describe what he perceived as the behavior of conservatives:

“”Reality is defined by a multimedia array of interconnected and cross promoting conservative blogs, radio programs, magazines, and of course, Fox News. Whatever conflicts with that reality can be dismissed out of hand because it comes from the liberal media, and is therefore ipso facto not to be trusted. (How do you know they’re liberal? Well, they disagree with the conservative media!) This epistemic closure ...

His usage is close to "false consensus" or "groupthink." He certainly doesn't mean they automatically know all the consequences of their ideas.
Sanchez's post kicked off the "epistemic closure" debate of 2010, a major internet event for political wonks. A number of pundits jumped into the fray, including Andrew Sullivan, Jonah Goldberg, David Frum, Ross Douthat, Bruce Bartlett, and Jonathan Chait.